Air refueling tankers utilize two basic methods for the transference of fuel from the tanker aircraft to the receiving aircraft. One method involves the tanker aircraft extending a drogue equipped flexible hose which the receiving aircraft plugs into via a refueling probe. For the most part, the hose and drogue is a passive system in regards to the tanker, in that once the tanker has extended the hose, no control inputs for drogue position are available from the tanker. This method is typically limited to fairly low flow rates of transference.
For higher fuel transfer rates, tanker aircraft typically employ a refueling boom. This refueling boom consists of a telescoping tube assembly which plugs into a receptacle on the receiving aircraft. All refueling booms in current use utilize airfoil control surfaces mounted on the refueling boom to move the boom throughout its operational envelope, e.g. up-down, side-to-side. The size and shape of these control surfaces are determined by the amount of force they must generate to overcome the boom aerodynamic drag. Current refueling booms typically have cross-sections which are elongated in the forward-aft direction, such that they expose a more streamlined shape to the airflow when the boom is trailing behind the tanker aircraft in flight, and the boom is located on the aircraft centerline. Side-to-side movement of the refueling boom exposes a profile with greater surface area to the on-coming airflow, thus increasing the aerodynamic drag on the boom, and as a consequence, the amount of force needed from the airfoil control surfaces to overcome that drag.
In order to mount the refueling boom to the tanker aircraft, various boom pivot configurations have been developed, e.g. alti-azimuth, canted axis, rolling axis, each of which attempt to minimize the refueling boom cross-section during movement in order to decrease aerodynamic drag. With all of these pivot configurations, however, a point is reached wherein the force generated by the airfoil control surfaces cannot further overcome the aerodynamic drag of the refueling boom due to the structural limitations of the refueling boom structure.